The temperature of an engine of a vehicle is increased to a high temperature by a great amount of heat of combustion generated in a combustion stroke of the engine. Thus, the vehicle is equipped with a cooling device that maintains the engine at an appropriate temperature. Such a cooling device is generally such that a coolant is supplied to the engine by a circulation flow channel and that the coolant discharged from the engine (discharge coolant) is cooled by a radiator.
Upon starting the engine, increasing the temperature of the engine as promptly as possible to an appropriate temperature (warm-up) is required in order to increase the combustion efficiency of the engine. The temperature of the engine can be promptly increased during the warm-up by circulating the discharge coolant in the engine without cooling the discharge coolant in the radiator. Thus, a circulation flow channel that circulates the discharge coolant to the radiator and a bypass flow channel that circulates the discharge coolant to the engine without circulating the discharge coolant to the radiator are provided. The circulation flow channel is provided with a thermostat for adjusting the temperature of the coolant supplied to the engine. The thermostat is a temperature adjusting valve that adjusts the temperature of the coolant supplied to the engine by being operated to be opened or closed according to the temperature of the coolant from the radiator. The thermostat is required to have high reliability for appropriately warming-up or cooling the engine.
A vehicle that diagnoses the thermostat is disclosed in Patent Literature 1. Specifically, a control device included in the vehicle measures the temperature of the coolant during the warm-up and diagnoses the presence of a malfunction of the thermostat on the basis of a relationship between the measured temperature and a threshold. A likelihood of an erroneous diagnosis exists in a case where the measured temperature of the coolant tends to be decreased, and thus, the vehicle stops diagnosing the thermostat.
A diagnostic device that diagnoses the presence of a malfunction of the thermostat, which is a temperature adjusting valve, on the basis of a measured value and an estimated value of the temperature of the discharge coolant is disclosed in Patent Literature 2. Specifically, the diagnostic device calculates a deviation between the measured value and the estimated value of the temperature of the discharge coolant and diagnoses the thermostat as having a malfunction in a case where the deviation is greater than a threshold.
The diagnostic device, in a case where a diagnosis is made that some malfunction occurs in the vehicle, informs a driver of the occurrence of a malfunction by, for example, turning on a warning lamp on a front panel.
One of items diagnosed by the diagnostic device is exemplified by a malfunction of the temperature adjusting valve. The temperature adjusting valve is a valve that is provided midway of a flow channel through which the coolant circulates between an internal combustion engine and the radiator of the vehicle. The temperature of the coolant is adjusted by switching the path of the flow of the coolant with the temperature adjusting valve. If, for example, an open failure occurs in such a temperature adjusting valve, the coolant is supplied to the radiator at all times, and the temperature of the coolant is excessively decreased. Thus, the diagnostic device is capable of determining whether or not the temperature adjusting valve is normally operated, on the basis of the temperature of the coolant.
A failure diagnostic device that diagnoses an electrically controlled thermostat valve, which is a temperature adjusting valve, is disclosed in Patent Literature 3. In a vehicle equipped with the failure diagnostic device, opening and closing operations of the thermostat valve are controlled in such a manner to match the temperature of the coolant circulated to the radiator and a target temperature. When the deviation between the target temperature and the actual coolant temperature is greater than a predetermined value during such control, a malfunction is determined to occur in the thermostat valve.
[Patent Literature 1] JP 2015-78657 A
[Patent Literature 2] JP 2000-104549 A
[Patent Literature 3] JP 2001-329840 A
A malfunction of the thermostat significantly affects cooling of the engine. Thus, a diagnosis of the thermostat is preferably performed after completion of the warm-up as well. In the related art disclosed in Patent Literature 1, a diagnosis of the thermostat is not permitted in a case where the measured temperature of the coolant tends to be decreased. When the warm-up of the engine is completed, the coolant circulates in the radiator and is supplied to the engine by operation of the thermostat, and the temperature of the coolant is changed on the basis of various causes. Therefore, if the thermostat is diagnosed simply on the basis of a change in the measured temperature of the coolant, the thermostat cannot be accurately diagnosed in a case where the temperature of the coolant is decreased by a cause other than a malfunction of the thermostat.
A malfunction of the temperature adjusting valve significantly affects cooling of the engine. Thus, a diagnosis thereof is preferably performed after completion of the warm-up as well. In the above related art, a diagnosis of the temperature adjusting valve is not permitted in a case where the measured temperature of the discharge coolant tends to be decreased. When the warm-up of the engine is completed, the discharge coolant circulates in the radiator and is supplied to the engine by operation of the temperature adjusting valve, and the temperature of the coolant is changed on the basis of various causes. Therefore, if the temperature adjusting valve is diagnosed simply on the basis of a change in the measured temperature of the discharge coolant, the temperature adjusting valve cannot be accurately diagnosed in a case where the temperature of the discharge coolant is decreased by a cause other than a malfunction of the temperature adjusting valve.
Among the items diagnosed by the diagnostic device, some item is permitted to be diagnosed only when the temperature of the coolant is greater than or equal to a predetermined temperature. Therefore, when the temperature of the coolant is decreased by, for example, an open failure of the thermostat, a part of the items remains not diagnosed.
If a diagnosis of the thermostat is not permitted as in the vehicle disclosed in Patent Literature 1 when the temperature of the coolant tends to be decreased, driving is continued while a determination such that a malfunction occurs in the thermostat is not made. As a result, the timing at which the temperature of the coolant reaches the predetermined temperature may be delayed with some malfunction not detected. In this case, since a failure diagnosis is not performed for a while, the timing at which a failure occurring in the vehicle is detected may be delayed.
When the temperature of the coolant is decreased, it is estimated that an open failure occurs in the temperature adjusting valve and that the coolant is excessively cooled by the radiator. However, a decrease in the temperature of the coolant may be generated by a cause other than an open failure of the radiator. For example, the temperature of the coolant may be decreased because a heating device is operated and consumes the heat of the coolant in order to heat the inside of the vehicle. Alternatively, the temperature of the coolant may be decreased because a heat generation amount of the engine is comparatively small. Therefore, an erroneous determination may be made if a determination such that an open failure occurs in the temperature adjusting valve is made at all times from a decrease in the temperature of the coolant.
In Patent Literature 1, a diagnosis of an open failure of the thermostat is disclosed as being not permitted in a case where the temperature of the coolant tends to be decreased, since a likelihood of an erroneous diagnosis exists. However, if a diagnosis is not permitted when the temperature of the coolant is decreased, a failure of the thermostat, if actually occurring at that time, remains not detected, and operation of the vehicle is continued.
The diagnostic device disclosed in Patent Literature 2 diagnoses the thermostat during the warm-up of the engine. However, since a malfunction of the thermostat significantly affects cooling of the engine, a diagnosis of the thermostat is preferably performed after completion of the warm-up of the engine as well.
However, the temperature of the discharge coolant after the warm-up is changed in a complicated manner on the basis of various causes such as the opening ratio of the thermostat and the operational state of the engine. Therefore, the measured value and the estimated value of the temperature of the discharge coolant may be changed by a cause other than a malfunction of the thermostat, and in this case, the thermostat cannot be accurately diagnosed.
A malfunction of the temperature adjusting valve, if occurring, does not immediately change the temperature of the coolant. The temperature of the coolant is changed after time elapses from the occurrence of a malfunction. Thus, in the failure diagnostic device disclosed in Patent Literature 1, the timing at which the occurrence of a malfunction in the temperature adjusting valve is detected is delayed, and informing the driver of a malfunction may be delayed.
Malfunctions occurring in the temperature adjusting valve include, for example, inoperability of a valve body of the temperature adjusting valve and a leakage of the coolant in the radiator due to, for example, a crack occurring in the valve body. That is, a malfunction may occur in the temperature adjusting valve due to various causes. However, the failure diagnostic device disclosed in Patent Literature 1 is not capable of determining a specific type of malfunction, though being capable of determining whether or not a malfunction occurs in the temperature adjusting valve.